Measuring the position and movement of small particles is essential in many scientific fields. Typical examples are microbead tracking and cell tracking in biomedical research. Microbeads are uniform polymer particles of about 0.3 to 500 microns in diameter, which can be used to absorb or couple with bio-reactive molecules such as proteins or nucleic acids. Microbeads are used to track the position of biomolecules in single molecule manipulation experiments. In such an application, microbeads are pre-coupled with a ligand, a biomolecule such as an antibody, streptavidin, protein, antigen, DNA/RNA or other molecule, which binds to the desired target molecule. The whole assembly can be manipulated precisely with a magnetic or optical field. An example of cell tracking is the sperm motility test, in which the count, shape, and movement of sperm cells are measured to evaluate the health status of the sperm, in order to provide information for infertility treatment.
In these applications, high accuracy of the position measurement of the particles and large field of view are two desirable specifications. The accurate position of the investigated particles can be computed to derive precisely the movement, trajectory, speed, acceleration or other behavior of the particles. The large field of view enables a large number of particles to be characterized, which can significantly enhance the statistics, precision, sensitivity, and certainty of the system. For instance, if thousands or tens of thousands of sperm can be characterized simultaneously in one motility test, more precise conclusions can be drawn from the evaluation.
Unfortunately, the concepts of high accuracy of the position measurement of the particles and large field of view often conflict in traditional imaging systems. To accurately calculate the position or movement of the particle, a magnified image of the particle is preferred. The larger the image of the particle, and the more pixels the image covers in the image sensor, the more precise can the position of the particle be calculated. However, the use of high magnification reduces the field of view to the size of the image sensor divided by the magnification ratio.
A lens-free technique is described in US20120248292A1 to obtain a large field of view with high-resolution image of small particles, but the investigated sample has to be located on the surface of the image sensor without any gap. As a result, the image sensor has to be replaced in each test, which is very costly and the imaging system is difficult to configure. For instance, most commercially available image sensors have a transparent window enclosing the sensor chip, and this window has to be removed to locate the sample at the surface of the chip. What is needed then is a system which is easy to build without the need to locate the sample adjacent to the image sensor, to obtain both high-accuracy position measurement of small particles and a large field of view.